OG 2.4.14

Limits on the Isotropic γ-ray / Cosmic ray Ratioin the Ultra High Energy region
from the EAS Experiment at Ooty
M. Sasano1, Y. Aikawa1, N.V. Gopalakrishnan2, S.K. Gupta2, Y. Hayashi1, N. Ito1, S. Kawakami1,
D.K. Mohanty2, K.C. Ravindran2, K. Sasaki1, K. Sivaprasad2, B.V. Sreekantan2, H. Tanaka1,S.C.Tonwar2, and K. Viswanathan2
1
Faculty of Science, Osaka City University, Osaka 558 8585, JAPAN2
Tata Institute of Fundamental Research, Mumbai 400 005, INDIA

Abstract

We have attempted to determine the isotropic γ-ray / cosmic ray ratio using the very large area(560m2)
muon detector operating with the EAS array at Ooty in southern India. The showers with their cores
incident within the area of the EAS array but with no associated penetrating track in the muon detectors,
classified as muon-poor showers, have been considered as primary γ-ray initiated shower candidates. We
have obtained upper limits on the flux of isotropic γ-rays over the energy region from 50 TeV to 550 TeV.
A conservative upper limit on the ratio, Iγ / ICR of 3*10-3 at 90% C.L., over the energy region, 200 TeV to500 TeV, has been placed.
60

1 Introduction:

West - East [m]It is important to estimate the40
gamma ray intensity apart from thetotal cosmic ray intensity in all energy
20
ranges for various studies inastrophysics. In the VHE/UHE region,0
it can be estimated by looking at theexcess of intensity from the direction-20of a point source which has been
confirmed by observations at other-40
wave lengths, if the astronomical
object exists at relatively short-60
distance. Gamma rays coming from-80-60-40-20020406080
sources at cosmological distances,such as distant AGN’s, and gamma
North - South [m]rays due to cosmological origin areFigure 1: The arrangement of EAS array
expected to arrive isotropically. In
2:1m S.C.(used detectors for this analysis
several tens of TeV region, theare crossed mark)isotropic gamma ray flux is expected
2:Muon detector (35m )to be higher as a result of cascadingfrom interactions of extremely highenergy cosmic rays with the cosmic microwave background radiation by the Bottom-Up model (Halzen etal, 1990) or the Top-Down model (Aharonian et.al., 1992). Though, it is experimentally very difficult todistinguish gamma-rays from cosmic rays with ground based observations, recently upper limits have beenplaced by several groups which seem to question the validity of such models in the several tens of TeV toseveral tens of PeV energy range (Matthews et al 1991, Karle et al 1995, Aglietta et al 1996, Chantell et al1997).

2 Detector:
The GRAPES III air shower experiment is located at Ooty in southern India (N 11.4, E 76.7 and 2200m
altitude). It is observing 2 components of air showers, the electro-magnetic component and the muoncomponent, to study cosmic gamma rays and particles in the several tens of TeV to several tens of PeVenergy range. 217 electron detectors (scintillation counters) are arranged within 64m of the Centre in a 8m
span hexagonal dense array. The scintillation counter consists of a 5cm thickness, 1m2 area plastic
scintillator viewed by a 5cm diameter photomultiplier. The muon detectors are arranged in the form of 16modules with the cluster located towards the northern edge of the electron detector array. Total muon
detection area is 560m2. A muon module consists of 232 proportional counters (cross-sectional area 10cm x
10cm and length of 6m) arranged in 4 layers separated by 15cm thick concrete layers under 2m thickness
concrete absorber giving 35m2 detection area for muons > 1GeV.

3 Data Analysis:

Number of Air Showers

The Ooty Air Shower Array started data taking with 100 scintillation counters and 16 muon modules
from April 1998 onwards with a trigger rate ~10Hz. We have analyzed 4-1/2 months data (April-September
1998) using signals from 80 scintillation counters which were operating satisfactorily and gave stable vaules
for single particle calibration. In this analysis, the electro-magnetic component data are used to estimate
the air shower energy and core location, and the muon component data are used to select potential
candidates for gamma ray primaries.
3.1 Electro-magnetic component: For the electro-magnetic component analysis, the following criteria
were used for off-line shower selection:(a) Total number of hit scintillation counters >=15.
(b) Out of the higher most 4 scintillation counters, outer most ring detector <=1.
(c) Sum of the particles of 4 scintillation
1400
counters >= 10 particles.
(d) Out of higher most 6 scintillation>30
1200
counters, except the farthest, diameter>50<= 30m.
1000>70
The core location, arrival direction, total>100
number of detected particles
800>200
for electro-magnetic component weredetermined for these showers. The core600
location is estimated by particle weighted
center of higher most 5 scintillation
400
counters. The arrival direction is estimatedby fitting a conical air shower front to the200
timing data. Additional selection criteria
were placed on the analysed showers as0follows:0246810121416(a) Core should be inside a trapezoid area
2as shown in Fig. 1.
Number of Detected Muons [/560m ](b) Zenith angle < 25 degs.
Figure 2: Multiplicity distribution of muons(c) Total number of detected particle is a cut parameter (>30, ----, >1300) related to energy.
(d) Distance from the center of the Muon Station-0 center to the core is a cut parameter (<30m, <90m)
related to the muon lateral distribution.
3.2 Muon component: For the muon component analysis, the detected muon number has been estimatedfrom the observed muon tracks in the 4 layers of the detector whose direction must match with the directionof the air shower arrival direction determined from data on the electro-magnetic component. The number ofdetected muons depends on the air shower primary energy, distance from the core to the muon detector, andthe muon detection area. In the Ooty air shower array, typically several muons can be detected for total
number of detected particle >30, distance from muon detector to core < 30m. Muon data recording gate
width is 5microsec per event, and the muon count rate, unrelated to air shower trigger, is ~3300/module/sec
for 4-fold coincidences. So, conservatively, the average number of muons due to chance coincidence isestimated to be ~0.25 per event for all muon detectors. In Fig.2, we show the muon multiplicity
distributions for different shower size groups classified by detected particle number.

4 Simulation and Calculation:
It has been known for a long time that gamma ray induced air showers are muon-poor compared toparticle/nucleus induced showers due to much less number of hadronic interactions. Monte Carlosimulations with COSMOS (Ver 4.92, K. Kasahara 1995) show that less than 5% muons are expected fromgamma ray induced air shower compared to proton induced showers for same primary energy. Since typicalevents observed by the Ooty air shower array have 0 to several tens of muons, we have classified no-muon
detected air showers as muon-poor events. We have calculated the upper limit on the ratio of gamma ray
flux to all cosmic ray flux using these muon-poor events as the candidates for gamma ray induced showers.
First, we estimate primary energy distribution and its median value by semi-Monte Carlo simulation
through the GENAS program (Ver 2.2, K. Kasahara & S. Torii 1990), using the same criteria for electron
detectors and event selection for both gamma ray and nucleus. Second, we input these energy distribution
data and core location distribution data into the Monte Carlo simulation (COSMOS Ver 4.92) to simulate
gamma ray induced air showers including trigger effect and errors in estimation of core position. Using
these results, we estimate the efficiency with which gamma ray induced air showers can be observed asmuon-poor events. Observation data and simulation results are shown in Table.
Table : Observation data and simulation results(<30m)
No. of Detected Electron [particle]>30>50>70>100
6176
93112
Median Energy for γ [TeV]Median Energy for CR [TeV]
47
62
78
99
No. of Air Shower
17239
12966
9946
7024
No. of Mu-poor Air Shower
40015475
21No. of Mu-poor U.L.(90%C.L.)
4331748728
Muon Detection Ratio for gamma [%]2.7
3.0
3.43.8
Muon Detection Ratio By chance [%]2323232375757474εγ [%]6.7E-02
3.1E-02
1.9E-02
7.6E-03
Upper Limit for Iγ / ICR (90% C.L.)

5 Discussion and Conclusion:
Upper limit on the ratio of gamma ray flux to the cosmic ray flux is given by:

Iγ
Nmuon − poor (90%C.L.) 1  ECR ≤⋅ ⋅εγ  Eγ ICRNall−α>200
175145
3169
13.9
4.7
2373
2.8E-03>300227
208199402.3
5.32373
2.0E-03

I /ICR Upper Limit (90% C.L.)

where Nmuon-poor(90%C.L.) is a 90%
1E+0
confidence level upper limit on thenumber of muon-poor air showers
HEGRA-AIROBICC
assuming Poisson distribution, Nall is
1E-1
Utah-Michigan
the total number of air showers, εγ is
EAS-TOPthe efficiency to detect gamma ray
CASA-MIA
induced air showers as muon-poor air
1E-2
Ooty, this work
showers, ECR is the median cosmic rayenergy, Eγ is the median gamma rayenergy, and α is the power-law
1E-3
spectral index for the primary energy
spectrum. The result is shown in Fig.
 1E-43 and compared with the upper limits
given by other groups.
It is seen from the Figure that theOoty result from the present work
1E-5
doesn’t improve the Iγ / ICR upper
10100
100010000
limit given by the HEGRA group for
Isotropic Gamma Ray Energy [TeV]
the 50 - 100 TeV region. However,
Figure 3: Upper limit of gamma ray/Icr for various group.
for the 200 - 550 TeV region, theOoty result gives a more strict upperlimit. In very near future, the numberof electron detectors in the GRAPES III array will increase to 217 and the gate width for the muon detectors
will be decreased from 5microsec to ~3microsec. These developments will make it possible for us to place abetter upper limit on the Iγ / ICR ratio for both the lower energy region as well as the higher energy range.

Acknowledgement
We are thankful to the Ministry of Education, Japan for partial financial support for this experiment. We arealso happy to acknowledge valuable contributions of S.T. Arasu, A.P. Amalaraj, G. Paul Francis,V.Jeyakumar, K. Manjunath, K. Ramadass, B.S. Rao, C. Ravindran, P. Sumathi, V. Viswanathan andT.Matsuyama during the installation, operation and maintenance of the instrumentation. The help andcooperation of the Radio Astronomy Centre for providing site facilities for the GRAPES III array are
gratefully acknowledged.

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